1. Field of the Invention
The present invention relates to a system and method responsive to a curve motion of a vehicle for controlling a steering reaction force imposed on a steering wheel of a vehicle.
2. Description of the Prior Art
Power steering systems have recently been adopted in many automotive vehicles. Such power steering systems add assistance torques through hydraulic boosting arrangements to steering forces applied to steering wheels by drivers and work to assist steering operations.
An electrically assisted power steering system has been proposed, as exemplified by a Japanese Patent Application Unexamined Open No. Sho 59-63264 published on Apr. 10, 1984, except the conventional hydraulically assisted power steering system.
FIG. 1 schematically shows the electrically assisted power steering system disclosed in the above-identified Japanese Patent Application.
In FIG. 1, a steering wheel 1 is connected to a first steering shaft 2. The first steering shaft is connected to a second steering shaft 5 via a first universal joint 4.
A direct-current (DC) motor DM is mounted on the second steering shaft 5 via a speed reducer 9. In addition, a steering torque sensor 8 is attached to the second steering shaft 5 for detecting a torque T.sub.c (hereinafter referred to as "steering torque") applied to the steering wheel 1. A third steering shaft 7 is connected to the second steering shaft 5 via a second universal joint 6. A lower end of the third steering shaft 7 is connected to a rack-and-pinion gear box 3.
It is noted that a gradient r between the first steering shaft 2 and second steering shaft 5 is set to be equal to the gradient r between the second steering shaft 5 and third steering shaft 7.
The above-described gear box 3 is connected to a tie rod 17. The tie rod 17 is connected to a knuckle arm 16 of a tire wheel 12.
A controller denoted by 10 receives an electrical signal indicating a detected value T.sub.c of the steering torque applied to the steering wheel 1 from the steering torque sensor 8 and controls a drive current I.sub.p sent to the servo motor DM so that the motor DM generates an assistance torque T.sub.A corresponding to the detected value T.sub.c of the steering torque. The conventional electrically assisted power steering system is mainly designed to reduce the steering force applied to the steering wheel 1 by the driver. Therefore, the driver, who receives turning (curve) motion information of the vehicle from a steering reaction force of the steering wheel 1, feels that he cannot obtain sufficient information on the turning motion.
In other words, the driver generally physically feels the steering reaction force when he operates the steering wheel and a change in a state variable of the vehicle's motion.
The driver evaluates an easiness in driving on the basis of these feelings.
In addition, the driver, in general, directs his attention toward a yaw rate of the vehicle in a middle and low vehicle speed regions and, in turn, toward a lateral acceleration in a high vehicle speed region.
In this way, although the driver requires such turning motion information as the lateral acceleration, or yaw rate, he relies only on his own feelings.